Types of tank structures of conventional LNG ships are roughly divided into a self-supporting spherical tank, a self-supporting prismatic tank (SPB) type, and a membrane tank. An independent spherical tank is a self-supporting spherical tank made of an aluminum alloy and is supported in a hold formed by a double hull, via a skirt extending from its equatorial portion. A thermal insulation work is done on an outer surface of the tank. Due to its spherical shape, the spherical tank has a disadvantage of not being capable of having a sufficient tank volume comparable to the size of the hull. The tank of this type little suffers damage even when its cargo sloshes at the time of heavy weather.
The self-supporting prismatic tank is one in which a prismatic tank is housed in a hold having a double hull structure. A cold insulator is provided on an outer surface of the prismatic tank made of an aluminum alloy. Strength members for the prismatic tank are provided on an inner side of the tank. This type requires void space between the prismatic tank and an inner hull, which accordingly reduces volumetric efficiency of the tank. On the other hand, having the strength members inside the tank, it has an advantage that sloshing of a liquid cargo does not easily occur.
Next, regarding the membrane type, on an inner surface of a hold having a double hull structure, thin sheets (membranes) made of nickel steel or stainless steel are affixed, with a cold insulator therebetween, to form a LNG tank. This type has an advantage that almost all the volume of the hold can be used as a tank volume. On the other hand, it has a disadvantage that the membranes and the cold insulator are likely to suffer damage due to the sloshing of a liquid cargo. It also has a problem that a thermal insulation work, in particular, the welding of the membranes is complicated and it requires a long period for the construction.